1. Field of the Invention
The present invention relates to an optical waveguide device.
2. Description of the Related Art
A branch optical waveguide device, having branch optical waveguide structures made of polymer materials, has advantages for providing significantly high productivity and low manufacturing costs, and, thus, such branch optical waveguide devices are used for component parts to fabricate optical modules.
In an optical communication network system based on optical fiber technology, optical fibers are installed from a station to individual homes. In the system, an optical fiber from the station is connected to a splitter module that has plural output ports. The ports connect plural optical fibers that are respectively delivered to individual homes. The optical signal communication provides two-way (bidirection) communication, where optical signals are delivered not only from the station, but from the individual homes.
The optical waveguide device is incorporated in the splitter module. Optical losses at ports of the module are required to be as uniform as possible. In general, infrared light having a wavelength of 1550 nm is used as the optical signal that is sent from the station to individual homes. An infrared light having a wavelength of 1310 nm is used as the optical signal that is sent from individual homes to the station. In actual communication, the optical signal is subjected to DWDM (Dense Wavelength Division Multiplexing), and infrared light having a predetermined bandwidth is used for the communication. Thus, a branch optical waveguide device is required to have a uniformity of optical losses for the predetermined bandwidth over all the ports. It will be required for the optical losses over all the ports to be more uniform in the future, because the communication band becomes wider in range with increases in picture delivery communications.
FIG. 11 is a plan view of a conventional 8-branch optical waveguide device 1.
As shown in FIG. 11, the 8-branch optical waveguide device 1 includes 8 ports P1 through P8, and incident light to a port Q propagates within a core pattern 10 and splits into 8 parts to send it out from the ports P1 through P8. The core pattern 10 includes seven branch points 12-1 through 12-7, a junction side core 11, the first stage branch side cores 21 and 22, the second stage branch side cores 31, 32, 33 and 34, and the fourth stage branch side cores 41 through 48. The core pattern 10 is formed to be axially symmetric at the center line CL drawn through the port Q.
When an optical signal is sent from a station to a home, the optical signal is incident to the port Q, propagates the junction side core 11, is divided at the branch points, and goes out through the ports P1 through P8.
FIG. 12 is an enlarged cross-sectional view of the optical waveguide device at the line A-A in FIG. 11 viewing along the arrows A.
As shown in FIG. 12, the 8-branch optical waveguide device 1 has an 8-branch optical waveguide provided on a semiconductor substrate 2. The 8-branch optical waveguide device is constructed on the silicon substrate 2 and includes a lower cladding layer 5 which is formed on the silicon substrate 2, the core pattern 10 which is formed on the lower cladding layer 5 (the second stage branch side cores 31, 32, 33 and 34 are indicated in the figure), and an upper cladding layer 6 which is formed on the lower cladding layer 5 and covers the core pattern 10.
FIG. 13 and FIG. 14 show simulation results of optical loss characteristics of a conventional 8-branch optical waveguide device.
FIG. 13 shows wavelength dependence on optical losses at ports P1 through P4.
Specifically, the lines LP1 through LP4 indicate the losses as a function of the wavelength at the ports P1 through P4. Further the losses at P5 through P8 are the almost similar to those of the ports P1 through P4.
In an ideal case, the optical loss for each port of the 8-branch optical waveguide device 1 is desired to be a constant amount, such as 9 dB, even for different wavelengths of incident lights. On the other hand, in practical cases, as shown in FIG. 13, the optical losses change as a function of wavelength of the incident light. Further, individual ports show different wavelength dependences of the optical losses between the ports.
FIG. 14 shows wavelength dependences on optical loss by branching and non-uniformity of optical loss at different ports.
In FIG. 14, solid square shapes indicate wavelength dependence of non-uniformity of optical loss at a port, and solid diamond shapes indicate wavelength dependence of the branching loss.
The non-uniformity of loss at a port is expressed by a differential between the maximum loss and the minimum loss at the predetermined port.
As shown in FIG. 14, the differential port loss changes, ranging from about 0.05 dB to about 0.4 dB, and at a wavelength of around 1450 nm, the differential port loss becomes larger, being 0.37 dB.
The reason for the non-uniformity of port loss is unclear. Optical signal (light) incident to the port Q (input port) propagates in the core pattern 10 with meandering for several reasons. It may be regarded that this meandering of light causes non-uniform light splitting at a branch point, so that the non-uniformity of optical loss takes place at the ports. In addition, it may be regarded that leaking light at the branch points 12-1 through 12-7 can be one of the reasons for the non-uniformity of branching loss.
As described above, the conventional optical waveguide device shows non-uniform port loss for a predetermined bandwidth, and the conventional waveguide device does not sufficiently meet the requirement for uniformity of optical loss over a wide range of wavelengths.
Reference 1: Japanese Patent Application Publication No. 7-92338